Hey guys, i've found a bunch of different info about how to level a floor. Even something about roofing felt.
We've got some 1/4" plywood and 1 - 1/2" plywood. The homedepot guy sold me some leveling cement, but then people said it would crack or be too heavy?

Felt paper is not going to level your floor, it could prevent cement from running through floor.. but I wouldn't be pouring that much cement in an attic.

How much is floor unlevel from end to end or side to side? Which way is it unlevel? Which direction do the floor boards in pic run ? Which way in relation to floor boards is it unlevel? Does the floor slope, or are there differences in height from 1/4" to 1 1/2 plywood?
Looks like a closet doorway in pic 1 ? Where are passage door ways? Will level
floor across door ways slope from side to side or be a transition step up or down?

You may need closly spaced furring strips across or paralell with joists over planks.

__________________

[CENTER][FONT=Comic Sans MS]Measure twice, cut once.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]Look at the nail, not the hammer. Watch the fence, not the blade.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]If you hook your thumb over your belt you won't hit it with the hammer or leave it layin on the saw table.[/FONT][/CENTER]

Hi that's the closet door the exit is directly across from it. It is bowing from the middle mostly on the right as seen on the photo. There are other boards underneath. The other room was also leveled with plywood. The right side is about 3/4" down. I think start there with 1/2 plywood. I guess glue/nail it down. Then use the cement leveler to fill the steps? Or try shim it i guess would creek too much?

Plywood has a grain.
Put 1.5" wide x 4' lg shims, of varying height of 3/4" and less, necessary to achieve level, each 16" across the plywood grain, 7 shims total for each plywood sheet.
Cutting these shims may prove challenging. A single shim may be 3/4" high at one end and zero at the other.

1000 lbs spread over 100 sq. ft is only 10 PSF of dead load. The same thickness of wood would give you maybe 300 lbs.

It sounds weird to put concrete in an attic but tiled bathroom floors have much the same configuration, a very dense, brittle material-then brittle or not underlayment-then wood.

The size and spacing of the joists under floor is what determines how much weight it can support. If by "bowing" you mean that floor is sagging, rather than a slope, it would be wise to check joists to see what you got.

Apart from the labor involved in removing existing flooring, it is easier to level a floor from joist level.

It is easier to put in furring strips across the slope than trying to cut long tapering ones. Glue and screw them thru existing floor into joists below.
Also check for any creaking from floor you have now. Screw loose flooring to joists.

If slope runs along joists; one end higher than other, or ends higher or lower than middle;
If the "right side" is 3/4" inch low I would put a 1 1/2" X 3/4" strip the full length or width of that side. You could use plywood or rip it off a 2x. Come 16" closer, to high side and measure height for next strip, rip it from 2X, and glue & screw it. Continue on untill you reach high side.

If slope is across joists put strips on top of joists, you may need some in between joists .

Glu and screw underlayment on strips to joists below. You mite want a light weight filler between strips as a sound deadener. This does not have to be as watery or heavy as self leveling cement as you can screed it level with a board dragged across strips. there are "concrete" mixes with light weight aggregates.

__________________

[CENTER][FONT=Comic Sans MS]Measure twice, cut once.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]Look at the nail, not the hammer. Watch the fence, not the blade.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]If you hook your thumb over your belt you won't hit it with the hammer or leave it layin on the saw table.[/FONT][/CENTER]

The bow is not that bad, i know what caused it.
I think i can leave it as is, the room is small already. Maybe put carpet or laminate tiles over it. Any ideas about filling the cracks first?
What kind of under pad should i use, cork?